There is a need to apply adhesive to electrochemical cell components, for example, portions of the inside surface of the casing for the cell. The portions of surfaces to be coated with adhesive can be very narrow or otherwise difficult to access using convention brushes or contact rollers. Many cells, such as conventional zinc/MnO2 alkaline cells include a plastic insulating plug which is inserted into an open end of the cell casing (housing) to seal the cell. There can be desirable benefits to applying adhesive sealant between the edge of such insulating plug and the cell casing, which is typically metallic. In such cells a metallic current collector in the form of an elongated nail is inserted through an aperture in the insulating plug so that the tip of the current collector passes into the anode mixture. It can be useful to apply adhesive sealant to the surface of the current collector or the insulating plug so that a tight seal develops when the current collector is inserted into the insulating plug. Conventional contact methods of applying the adhesive, for example, with brushes or rollers are usually slow or are difficult to apply to very narrow or difficult to reach surfaces.
Zinc/air depolarized cells are typically in the form of miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. There can be a problem of leakage of electrolyte from such cells if they are not propely sealed, particulary if the cell is misused. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each have a closed end an open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the cathode casing is typically inserted over the open end of the anode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising zinc, usually particulate zinc, with mercury optionally added to reduce gassing. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The closed end of the cathode casing (when the casing is held in vertical position with the closed end on top) can have a raised portion near its center or a flat bottom. This portion forms the positive terminal and typically contains a plurality of air holes therethrough. Cathode casings with a raised center on the closed end usually have an integrally formed annular recessed step, which extends from and surrounds the raised positive terminal.
The catode casing contains an air diffuser (air filter) which lines the inside surface of the raised portion (positive terminal contac area) at the casing's closed end. The air diffuser is palced adjacent to air holes in the raised portion of the casing closed end. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. The cathode material can be part of a cathode catalytic assembly which is inserted into the cathode casing so that it covers the air diffuser (filer). The cathode catalyc assembly can be formed by laminating a layer of electrolyte barrier material (hydrophobic air permeable film), preferably TEFLON polymer (tetrafluoroethylene), to one side of the catalytic material and an electrolyte permeable (ion permeable) separator material to the opposite side. The cathode catalytic assembly is then typically inserted into the cathode casing so that its central portion covers the air diffuser and a portion of the electrolyte barrier layer rests against the inside surface of the step.
In high drain or other demanding services, electrolyte can migrate to the edge of the catalytic cathode assembly and leakage of electrolyte from the cathode casing can occur. The leakage, if occurring, tends to occur along the peripheral edge of the cathode catalytic assembly and the cathode casing and then gradually seep from the cell through the air holes at the cathode casing closed end. The potential for leakage is also greater when the cathode casing is made very thin. For example, having a wall thickness of between about 4 and 10 mil (0.102 0.254 mm) or lower, for example, between about 2 and 6 mil (0.051 and 0.152 mm) in order to increase the amount of available internal volume. There is a greater tendency for the thin walled cathode casing to relax after crimping closes the cell. Such casing relaxation can result in the development or enlargement of microscopic pathways between the cathode catalytic assembly and the inside surface of cathode casing step, in turn providing a pathway for electrolyte leakage.
In commonly assigned U.S. Pat. No. 6,436,155 B1 a pad transfer method is disclosed for applying a narrow band of adhesive to the recessed annular step surrounding the raised terminal portion of the cathode casing of a zinc/air cell. The application of adhesive to the inside surface of the recessed step provides a tight seal between the cathode assembly and cathode casing of a zinc/air cell. The adhesive applied by pad transfer method prevents leakage of electrolyte around the edge of the cathode assembly and thus prevents electrolyte from escaping through air holes in the cathode casing.
In commonly assigned U.S. Pat. No. 6,610,353 B1 a jet spray method is disclosed for applying adhesive to electrochemical cell components and casing. The nozzle assembly comprises a nozzle having a resilient glass capillary tube surrounded by a piezoelectric transducer. The piezoelectric transducer causes the capillary tube to pulse (vibrate) thereby causing liquid adhesive passing therethrough to dispense from the nozzle in the form of micro sized droplets. In particular the jet spray method allows application of very narrow band of adhesive to the recessed step surrounding the raised terminal portion of the cathode casing of a zinc/air cell.
Another method which has been used to dispense fluids, including liquid adhesives, is through use of a needle valve applicator. Such method employs a pneumatically actuated needle valve which allows fluid to pass through a tubular tip connected to the valve. Needle valve applicators have been used to apply adhesive in globs to specific portions of cell components or specific regions of the cell casing. They have been generally considered ineffective for accurate application of very narrow continuous bands of adhesive, e.g. band widths less than about 30 mil (0.76 mm), particularly when the application path is curvilinear. Representative needle valve applicator patents are: U.S. 2001/0052585A1; U.S. Pat. Nos. 6,244,522 B1; 6,164,568; 5,277,344; 5,078,325; 4,988,015; 4,922,852; and 4,579,255.